1. Field of the Invention
The present invention relates to the field of gas turbine engines and, in particular, to that of the combustion chambers of such engines.
2. Description of the Related Art
The combustion chamber of a gas turbine engine receives compressed air from an upstream high-pressure compressor and provides a gas that is heated by combustion in a combustion zone supplied with fuel. The chamber thus comprises a chamber end wall situated upstream and to which the various fuel injection systems are attached. FIG. 1 shows a chamber of the prior art. The annular chamber 1 is housed inside an engine casing 2 downstream of the compressed air diffuser 3. It comprises an interior wall 4 and an exterior wall 5 between them delimiting a combustion zone. In its upstream part, the chamber comprises a transverse chamber end wall 6 on which openings are formed, each opening being equipped with a carbureted-air supply system 7. Such a system is supplied with fuel from a liquid-fuel injector and comprises concentric cascades of vanes to create streams of air that swirl, encouraging them to mix with the layer of atomized fuel.
Some of the air from the diffuser is diverted away from the fuel intake zone by the fairing 8 and flows along and around the outside of the exterior wall and along and around the outside of the interior wall.
The proportion which passes along inside the carburetion zone, crosses the chamber end wall 6 and the mixture is ignited by sparkplugs arranged on the exterior annular wall. The primary combustion zone is therefore situated immediately downstream of the chamber end wall. Deflectors 9 made of a metallic material line the inside of the chamber end wall and their function is to protect it from the intense radiation produced in the primary combustion zone. Air is introduced through orifices made in the chamber end wall behind the deflectors in order to cool them. This air flows along the rear face of the deflectors and is then guided so that it forms a film along the longitudinal exterior walls of the chamber.
Because the chamber end wall deflectors are not mechanically stressed, have no structural role and their only function is to afford thermal protection, and with a view to optimizing the air flows, it would be desirable to be able to reduce the stream along the chamber end wall and assign part of it to another function, notably that of cooling the interior or exterior walls.
Also, increasingly improved engine performance leads to increasingly high chamber temperatures being sustained. In order to conform to chamber life specifications, it would be necessary to intensify the cooling of the chamber walls and of the chamber end wall deflector. The solution involving increasing the cooling flow rate would be detrimental to chamber efficiency.
In order to solve this problem, the proposal is for the known metal deflector to be replaced with a CMC (ceramic matrix composite) deflector. The high-temperature capability of this material is far better than that of metal. This solution will make it possible to control the flow of deflector cooling air and, for the same chamber operating temperature, reduce it, so that a proportion of it can be reassigned to some other function or, alternatively, to allow higher operating temperatures to be tolerated for the same cooling air flow.
CMCs, ceramic matrix components, are known per se. They are formed of a carbon fiber or refractory reinforcement and of a ceramic matrix. The manufacture of a CMC involves producing a fibrous preform intended to constitute the reinforcement of the structure, and densifying the preform with the ceramic material of the matrix. CMCs have the advantage of maintaining their mechanical properties up to high temperatures in an oxidizing environment.
Fitting a component of this type in a metal structure does, however, present difficulties notable because of the substantial difference in their expansion coefficients. A CMC has a thermal expansion rate that is one quarter of that of the metal used for the chamber. Moreover, this material can be neither welded nor brazed.